MEMBRANE LATERAL COMPRESSIBILITY DETERMINED BY NMR AND X-RAY-DIFFRACTION - EFFECT OF ACYL-CHAIN POLYUNSATURATION

The elastic area compressibility modulus, K-a, of lamellar liquid crys talline bilayers was determined by a new experimental approach using H -2-NMR order parameters of lipid hydrocarbon chains together with lame llar repeat spacings measured by x-ray diffraction. The combination of NMR and x-ray techniques yields accurate determination of lateral are a per lipid molecule. Samples of saturated, monounsaturated, and polyu nsaturated phospholipids were equilibrated with polyethylene glycol (P EG) 20,000 solutions in water at concentrations from 0 to 55 wt % PEG at 30 degrees C. This procedure is equivalent to applying 0 to 8 dyn/c m lateral pressure to the bilayers. The resulting reductions in area p er lipid were measured with a resolution of +/-0.2 Angstrom(2) and the fractional area decrease was proportional to applied lateral pressure , For 1,2-dimyristoyl(d54)-sn-glycero-3-phosphocholine, stearoyl(d35)- 2-oleoyl-sn-glycero-3-phosphocholine (SOPC-d(35)), and d35)-2-docosahe xaenoyl-sn-glycero-3-phosphocholine (SDPC-d(35)) cross-sectional areas per molecule in excess water of 59.5, 61.4, and 69.2 Angstrom(2) and bilayer elastic area compressibility moduli of 141, 221, and 121 dyn/c m were determined, respectively. Combining NMR and x-ray results enabl es the determination of compressibility differences between saturated and unsaturated hydrocarbon chains. In mixed-chain SOPC-d(35) both cha ins have similar compressibility moduli; however, in mixed-chain polyu nsaturated SDPC-d(35), the saturated stearic acid chain appears to be far less compressible than the polyunsaturated docosahexaenoic acid ch ain.